Abstract: The morphology and classification of coelomocytes are considered as the foundations for study on immune system of sea cucumber Apostichopus japonicus. In this study, the results of coelomocytes staining were compared using different dyes including Giemsa, Wright's, Wright's - Giemsa at different conditions ( fixed time, culture time, staining time and temperature, and decoloration method and time). The perfect staining method of coelomocytes in the sea cucumber was obtained on the criteria of the staining of granules, nuclei, cytoplasm and pseudopodium and the definition of cell outline, which included cell drop technique, incubating in wet chamber for 15 - 30 min at 25 ℃, fixing in methanol for 1 -2 min, staining in φ(Wright's) : φ(Giemsa) = 1:1 for 1 -2 min, washing in water and sealing up with colophony.